Fungicides are most warranted for FHB control when wet, rainy conditions occur during anthesis and early grain fill. However, rainfall following treatment application may affect efficacy against FHB and DON, and this effect may vary among fungicides. In this study, we determined the rainfastness of the fungicides Prosaro (prothioconazole + tebuconazole), Caramba (metconazole), and Miravis Ace (pydiflumetofen + propiconazole) when applied to wheat spikes at anthesis in three field experiments conducted in 2020, 2021, and 2022. The experimental design was a randomized complete block, with a split-plot arrangement of simulated rainfall treatment (combinations of durations [0, 15, 30, 60, and 120 min] and start times [0, 15, 30, and 60 min] after fungicide application) as whole-plot and fungicide treatment (Prosaro, Caramba, and Miravis Ace) as sub-plot. The three fungicides were applied to separate plots at their respective label-recommended rates of 6.5 (Prosaro), 13.5 (Caramba), and 13.7 (Miravis Ace) fl. oz./A, with or without the nonionic surfactant Induce (0.125 v/v). Irrigation risers mounted in each whole plot were then used to simulate the desired rainfall treatment at an intensity of 6.5 mm/h. Approximately 24-36 h after the treatments were applied, all plots were spray inoculated with a spore suspension of F. graminearum. Results were fairly consistent across experiments and among fungicide treatments within experiments. On average, when applied with Induce, fungicide treatments reduced FHB index (IND) by 71 to 87% and DON by 62 to 70%, with the performance of the three fungicides being fairly consistent across rainfall treatments. For DON, percent control relative to the nontreated check (C) ranged from 58 to 73% across rainfall durations and 60 to 73% across rainfall start times. However, efficacy was substantially lower (C < 51% for both IND and DON) when the fungicides were applied without the surfactant and plots were subjected to 120 min of simulated rainfall, beginning immediately after the treatments were applied. These results suggested that Prosaro, Caramba, and Miravis Ace are very rainfast once applied with Induce. Data from the three experiments will be pooled and analyzed to formally quantify fungicide and rainfall treatment effects.

ACKNOWLEDGEMENTS AND DISCLAIMER: This material is based upon work supported by the U.S. Department of Agriculture, under Agreement Nº 59-0206-4-018. This is a cooperative project with the U.S. Wheat & Barley Scab Initiative. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the authors and do not necessarily reflect the view of the U.S. Department of Agriculture.